And, counter-intuitively, spring is the season in which we are in shortest supply of the vitamin.

"People run out [of their vitamin D stores] as the year progresses and reach a low point in spring," says study author, Professor Steven Boyages from the University of Sydney. “Even by December, the first month of summer, levels were still 46 per cent below their peak. Although levels of ultraviolet-B radiation, the body’s principal source of vitamin D, would be rising by then, this reflects the time it takes to replenish the body’s stores.”

While Boyages is surprised by the extent of the problem and the sharp rise in spring, the results may be explained by the fact that "these days we're all protecting ourselves against the sun, working longer days and working indoors."

Vitamin D is best absorbed by the body through sunshine, but of course with too much sun come the widespread and devastating consequences of skin cancer. Vitamin D is also found in milk, eggs and fatty fish such as herring, mackerel, sardines and tuna. Deficiency can cause brittle bones and has also been linked (sometimes tenuously) to mental health problems and diseases such as cancer, diabetes and multiple sclerosis.

The issue has received much exposure over the past few years as our D levels seem to be dropping as quickly as rates of skin cancer are rising. Weighing up sun exposure versus vitamin D deficiency "is a case of too much of either is no good and too little is no good," Boyages says. "We don't really understand why [there is a parallel] - it's a complex issue."

Australians are spending around $100 million a year on vitamin D testing while sales of the supplement are soaring - it is worth more than $150 million a year in Australia alone. But, popping a few pills may not be the answer.

Studies looking at the benefits of supplementation have been largely inconsistent. One British study, in the October issue of The Journal of Clinical Endocrinology & Metabolism, has found vitamin D supplements do not protect against cardiovascular disease, while other recent studies have found too much supplementation could increase inflammation in the body.

"Most of the supplements are pretty safe," Boyages says. "[But] we have to be careful about fixing something before we fully understand it … and of making associations … Some [studies] have drawn interesting conclusions."

He would like to see more long-term, randomised trials, specifically looking at the effect of vitamin D supplementation on bone density, brain function and diabetes. On the proviso of further research, Boyages is also a proponent for fortifying our food supply with vitamin D, as Professor Caryl Nowson, of Deakin University called for earlier this year. But, primarily, he believes the answer lies in taking a holistic approach. Apart from insufficient sunshine, people who are overweight or have poor diets are at a greater risk. The implication is that our habits are harming our health.

We need to get out more. "We need more sun at the end of winter and early spring and less time in the peak of summer," Boyages says. But, we also need to address our sedentary lifestyles and our diets. The increases of such problems, Boyages says "are telling us something about how we live our lives. We need to find the right balance and look at ourselves."